1. Field of the Invention
The present invention relates to a method and apparatus for locating moving or stationary objects and, more specifically, to a method and apparatus for locating the distance of an object to a reference point and/or the angle of an object to a reference point for objects within a large range and with high accuracy through the use of reflected optical radiation comprising a plurality of different wavelengths.
2. Description of the Related Art
Various laser interferometric techniques have been utilized to measure the distance to an object. One of these techniques involves analyzing the fringes of an interference pattern created by the intersection of light waves transmitted to and reflected off an object. Although these fringe analyzing techniques can be highly accurate, their absolute range is limited to the ambiguity length of the system, i.e., the wavelength of light. Therefore, the distance to an object cannot be determined beyond the ambiguity length.
Fringe analysis, however, can be used to measure the distance to an object beyond the ambiguity wavelength by continuously directing light waves onto an object and counting the number of interference fringes produced by the intersecting transmitted and reflected waves as the object moves. However, only relative and not absolute distances can be measured with this technique. In addition, if the light waves incident on the object are interrupted, even the relative distance information is lost.
In lieu of these fringe counting techniques, multiwave techniques, wherein the phase differences are measured between a plurality of transmitted and respective reflected light waves of a different optical wavelength have been utilized to calculate absolute distances within a large range and with high accuracy. In one such method, a plurality of optical waves each having a different wavelength are sequentially reflected off an object. The phase differences between the transmitted waves and respective reflected waves are then sequentially detected and analyzed. If the object moves, however, even by an amount equal to a portion of an optical wavelength, the method of sequential phase measurements will be invalid and the measured distance will be in error. Therefore, only the distance to an object which is stationary can be determined by this sequential multiwave technique (see, Williams, C.C. and Wickramasinghe, H.K., "Optical Ranging by Wavelength Multiplexed Interferometry", Journal of Applied Physics, Vol. 60, No. 6, pp. 1900-1903, Sept. 5, 1986; Beheim, G., "Fiber-optic Interferometry Using Frequency Modulated Laser Diodes", Applied Optics, Vol. 25, No. 19, pp. 3469-3472, Oct. 1, 1986; and Kikuta, H., Iwata, K., and Nagata, R., "Distance Measurement by the Wavelength Shift of Laser Diode Light", Applied Optics, Vol. 25, No. 17, pp. 2976-2980, Sept. 1, 1986).
In another multiwave technique, two optical waves of distinct wavelengths and polarized at 90.degree. to each other are directed onto an object. The phase differences between the transmitted waves and the respective reflected waves are then simultaneously detected. Before the phases can be analyzed, however, two signals created by the intersecting waves must be separated. Because, at most, only two polarizations can be separated at any one time, this method is limited to the simultaneous measurement of only two phases, i.e., the phase difference between the first transmitted optical wave and the respective reflected wave, and the phase difference between the second transmitted optical wave and the respective reflected wave. In addition, this method is limited to locating objects which are very smooth because when a light beam is reflected off a rough surface its polarization changes. This unwanted change in polarization produces crosstalk between signals thereby reducing the accuracy of the system. Thus, the polarization technique is limited to slowly moving and very smooth objects (see, den Boef, A.J., "Two-Wavelength Scanning Spot Interferometry Using Single Frequency Diode Lasers", Applied Optics, Vol. 27, No. 2, pp. 306-311, Jan. 15, 1988).
In another multiwave technique, two optical waves of different wavelengths are directed onto an object and their reflection is detected. The two wavelengths are mechanically separated by a dispersion prism and their phases are measured. In practice this method is limited to the simultaneous detection of just a few wavelengths since only a few wavelengths can be mechanically separated at any one time. Also, the wavelengths of the optical waves cannot be changed or tuned without modifying the optical hardware (see, A.F. Fercher, H.Z. Hu, and U. Vry "Rough Surface Interferometry with a Two-Wavelength Heterodyne Speckle Interferometer", Applied Optics, Vol. 24, No. 14).